Searching grey literature is an important part of a systematic review strategy. Common sources of grey literature can include government agencies, advocacy groups, private foundations, and industry, trade, or professional organizations. The types of grey literature you find might include conference abstracts and proceedings, theses and dissertations, government publications, regulatory data, and stakeholder reports such as white papers, working papers, and internal documentation.
When considering a search for grey literature, an index such as GreyGuide can be a good place to find suggestions of where to get started.
Structured Google searching can also be powerful tool in locating relevant grey literature. Reach out to a librarian for help developing a grey literature search strategy.